1. Field of the Invention
The present invention relates to a mobile radio system and a mobile radio control method for effectively executing radio communications among mobile stations and radio communications between a fixed station and the mobile stations. Preferably, the mobile stations are provided with a terminal such as a personal computer or the like.
2. Description of the Related Art
A variety of communication systems are already known and various multiplexing communication systems have been proposed with requirements for large capacity transmission. For example, TDMA (Time Division Multiple Access), FDMA (Frequency Division Multiple Access), CSMA (Carrier Sense Multiple Access), and CDMA (Code Division Multiple Access) or the like are known and are also applied to radio communications. Moreover, a system formed by combining these multiplexing communications is also known for realizing radio communications among many mobile stations or between a fixed station and a plurality of mobile stations.
Moreover, in a mobile phone system, a plurality of base stations are distributed for arrangement to partially provide overlapped service areas, enabling communications with the other mobile phones or fixed phones, while a user carrying a mobile phone is moving, and assigning the channels to each mobile phone from the base station side. Moreover, a control method has also been proposed, in which a mobile station issues a channel assignment request to a fixed station with a CSMA/CA system from a mobile station using the common channel. The fixed station executes the scheduling of transmission time based on the TDMA system. (For example, refer to Japanese published patent application JP-A No. 2002-374265). Moreover, a system and a control method enabling the ensuring of bandwidth have also been proposed (for example, refer to Japanese published patent application No. 2005-73240). In this system, a beacon region for transmitting respective beacon packets from a plurality of master stations (fixed stations), a TDMA region for using the band in which the permitted particular station is assigned on the time division basis, and a CSMA region enabling the access through competition are periodically divided on the time axis, and region selection is conducted.
For radio communication on a 1:1 basis by a plurality of mobile stations, assignment of communication channels is required to avoid mutual interference within the predetermined frequency band in which a plurality of communication channels are set. When each fixed station (base stations) is provided for each area to cover almost every service area as in the case of a mobile phone system, assignment of communication channels may be controlled for mobile stations (mobile phones) on the side of the fixed station (base station), but when a plurality of fixed stations are distributed to places of interest along the roads or the like, or when fixed stations defining an event area as the service area are allocated, if a mobile station does not move to the service area (hot spot) of such fixed station, it is impossible to enjoy the communication channel assignment service. Therefore, a system for self-controllably selecting the communication channel to avoid mutual interference by introducing the CSMA system can be adapted for radio communication among the mobile stations.
Moreover, it is also possible to use a CSMA system as a multiple connection system among mobile stations and a TDD (Time Division Duplex) system as a multiplex system of radio communications between a mobile station and a fixed station. In such radio communication systems, when a mobile station comes close to the fixed station, the transmission timing of the mobile station is determined in accordance with the TDD slot timing, and when a mobile station makes communication with the fixed station, the TDD slot timing is synchronized. However, if the fixed station does not exist in the area near a mobile station, the mobile station is required to continuously maintain a timing to be self-controllably synchronized with the fixed station, unlike the cellular radio system in which the slot timing signal can be acquired from the fixed station.
Accordingly, it is also considered to adapt a means for acquiring synchronization by obtaining accurate time information based on the GPS (Global Positioning System) for the slot synchronization, but continuous reception of radio waves from the four or more GPS satellites is always required in order to continuously acquire time information by receiving the radio wave from the GPS satellite. However, it is difficult for a mobile station, in the area wherein the radio wave receiving state from a GPS satellite is not so good, to obtain time information through radio wave reception from a GPS satellite. Therefore, continuation of self-controllable synchronization becomes difficult in some cases. Moreover, when the fixed station does not exist at an area near mobile stations, and the radio wave receiving state from a GPS satellite is rather bad, it is possible to realize radio communications with a CSMA system among the mobile stations. However, for realization of highly efficient radio communication by equalizing, in accordance with the TDD system, the slot time length of the time slot used in the CSMA system to the slot time of the time slot designated with the TDD slot timing used by the fixed station, slot synchronization must be continuously attained by receiving the radio wave from the GPS satellite and then obtaining the accurate time information. In this case, a mobile station often faces, as explained above, difficulty in obtaining the time information by always seizing four or more GPS satellites. Therefore, there arises a problem that continuous acquiring of accurate slot synchronization becomes difficult, resulting in interference on radio communications of the other mobile stations.